A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
While contamination control is an issue for any lithographic apparatus, including conventional transmissive lens based systems, and especially for systems generally referenced as EUV (Extreme UltraViolet) systems, operating on shorter wavelengths operating below 20 nm, contamination control of mirror systems is desirable due to the sources that have a challenging contamination control. Reflection of EUV radiation uses specific mirror design with advanced materials. The reflectivity of these mirrors influences the EUV transmission of this imaging system. The interaction of EUV light with CxHy molecules at or near the surface of these mirrors causes deposition of carbon at the mirror surface. The latter significantly affects the reflective properties of the mirrors and thus it affects the EUV transmission of the imaging optical system.